1. Field of the Invention
This invention relates generally to optical sensors for use in receiving signals having wide variations in intensity and in particular to such a sensor having a rapid gain variation to accomodate the changes in signal intensity.
Laser radar (LIDAR) and other time/range to target applications have an inherent problem because the intensity of the returning signal has extreme variations. For a distant target the returning signal is of such low intensity that it requires amplification in order to be resolved by the photo sensor. On the other hand, amplification must be reduced for the high intensity signal returning from a closer target. Because such devices are usually used in a scanning application, a rapid change in gain is needed.
2. Description of the Prior Art
One approach to solving the range dependent laser radar dynamic range problem previously used graded intensity target illumination beams to direct more energy toward distant targets and less energy toward near targets. This technique suffers from slant path/platform dependent geometrical restrictions that make it difficult in near ground level applications to provide range independent uniform optical signal returns.
A second approach involved voltage control of the gain of standard gatable Intensifier Silicon-Intensifier Target (ISIT) camera tubes and Silicon-Intensifier Target (SIT) camera tubes. This technique fails in that the control of the gain of devices such as the ISIT and SIT also affects device focus. The control of both focus and gain then becomes much more complex and also must be done at extremely high working voltages (up to 20,000 VDC).
Hess, U.S. Pat. No. 3,557,373, entitled "Method and Circuits for Regulating the Gain of a Photomultiplier for Laser-Telemetry Apparatus", sets the gain of the photomultiplier as a function of the intensity of the prevailing background radiation.
Gebel, U.S. Pat. No. 3,652,154, entitled "Light Control System for Use in Very Low Light Intensities", provides a relatively constant light intensity to the image receiving device in spite of changing scene intensities through the use of a storage light detector which integrates temporally the light intensity from the scene viewed.
Wyess, U.S. Pat. No. 3,666,957, entitled "Brightness Limiter for Image Intensifiers", relates to an image intensifier having a microchannel plate (MCP). The system has an output screen in the proximity of the output of the MCP. The current supplied to the output screen which is proportional to brightness is sensed, and the voltage applied across the MCP is changed to limit the brightness to a desired level.
Macknik et al, U.S. Pat. No. 3,678,279, entitled "Automatic Variable Gain Optical Tracker for Space Vehicles Comprising D-C System for Error Signal Generation", disclose a quadrant target tracker having a separate photomultiplier for each quadrant. The outputs of the photomultipliers are summed and used to provide automatic gain control of the photomultipliers which varies inversely with the brightness of the target.
Ramsay et al, U.S. Pat. No. 3,694,659, entitled "Automatic Control Circuit for Image Intensifier", control the brightness of a multistage image intensifier tube to suppress the normal response to intense flashes of light by, e.g. monitoring direct current changes in the oscillator supply circuit and limiting current in this circuit when it is excessive.
Kreda, U.S. Pat. No. 3,714,441, entitled "Photomultiplier Gain Control Circuit", directs a reference light beam on a photomultiplier to establish a desired output voltage. This voltage is compared with that resulting from incident light and the voltage supplied to the photomultiplier is increased or decreased accordingly.
Chow, U.S. Pat. No. 3,816,744, entitled "Fast Response Automatic Brightness Control Circuit for Second Generation Image Intensifier Tube", discloses a protection circuit for an image intensifier tube which senses screen current at the gate electrode of a junction FET. A low junction capacitance diode is used in the circuit to reduce the time for response to changes in the screen current.
Lawrence et al, U.S. Pat. No. 3,864,595, entitled "Automatic Brightness Control for Gated Micro-Channel Plate Intensifier", use a gating circuit to apply the electron image produced in the photocathode element to the micro-channel plate only when desired. A brightnesss control circuit is used to prevent saturation of the phosphorus screen.
Moyers, Jr., U.S. Pat. No. 3,941,999, entitled "Automatic Focus Pulse Gated System", uses a toroidal, transparent member of changing thickness to vary the focal length of the objective lens of the system. As the toroidal member is rotated, the gating pulse timing is also changed.
D'Agostino, U.S. Pat. No. 3,980,880, entitled "Automatic Exposure Control Circuit for an Image Intensifier Camera", limits the energy provided to an image intensifier tube to that obtained in the discharge of a capacitor.
Orlando et al, U.S. Pat. No. 3,984,728, entitled "Image Intensifier Tube Gating Circuit", disclose a gating circuit which eliminates the voltage between the image intensifier tube anode and cathode when the tube is cut off. This eliminates scintillations which cause distortions on photographic film.
Moore, Jr. et al, U.S. Pat. No. 4,044,249, entitled "Voltage Supply Including Bilateral Attenuator", disclose circuitry for producing a plurality of DC voltages which requires only a single oscillator.
Loty, U.S. Pat. No. 4,100,406, entitled "Photoelectric Shutter Tube with Microduct Wafer Incorporated in a Wave Propagation Line Which is Integrated in said Shutter Tube", is concerned with matching the propagation in a control signal line with that in the shutter tube.
Fouilloy, U.S. Pat. No. 4,195,222, entitled "Power-Supply Device for a Microchannel Tube", discloses a two level microchannel plate voltage control to eliminate hunting in the device.
French, U.S. Pat. No. 4,226,529, entitled "Viewing Systems", describes a range gated laser scene illuminated arrangement utilizing time gating to view different ranges in the scene.
Heppner et al, in an article in The Review of Scientific Instruments, Vol. 41, No. 10, October 1970, entitled "Blanking Method for Continuous Channel Electron Multipliers", describe a gating system for a channel electron multiplier.